<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    // 变量声明/赋值
    var a, b, c;
    var num = 10, num2 = 10,num3 = 30;
    function isNull () {
        return '';
    }
    // 判断是否存在某全局函数/变量
    console.log("isNull" in this);
    console.log('num' in this);
    // 当值为假时，不能用下述方法判断
    console.log(this.isNull());
    console.log(this.num);
</script>
</body>
</html>